A flame scanner monitors the combustion process in a fossil fuel fired combustion chamber to provide a signal indicating the presence or absence of a stable flame. With the presence of a stable flame it is safe to continue feeding fossil fuel into the combustion chamber of the steam generator. In the event that the flame becomes unstable, or the flame is lost completely (known as a flame out condition), the flame scanner provides a loss of flame signal. Based upon a loss of flame signal, fossil fuel delivery to the combustion chamber can be discontinued before an unsafe operating condition develops. In some systems, a human operator interrupts the fuel supply based upon the loss of flame signal; in other systems a burner management system (BMS) interrupts the fuel supply based upon the loss of flame signal.
Conventional flame scanners produce an electrical signal based upon a monitored flame. This resulting analog electrical signal is transmitted to processing electronics that are housed separately from the flame scanner, typically in an equipment rack located adjacent to a control room. The strength of the produced signal is typically proportional to the intensity of the monitored flame. If the signal strength falls below a lower set point, or raises above an upper set point, delivery of main fuel into the combustion chamber is interrupted. Set points are sometimes referred to as trip points.
The signal path from each flame scanner to the processing electronics is via a double-shielded cable, which typically includes five conductors. Because of the size of each double-shielded cable as well as the number of double-shielded cables, one being required for each flame scanner, a considerable amount of space is necessary for routing cable bundles to the processing electronics. Additionally, because of the type and number of cables required, high initial capital outlay costs are required. Accordingly, a need exists for a flame scanner having fewer and less expensive cabling requirements.
One type of flame scanner is an ultraviolet tube flame scanner which produces a pulsed electrical output whose pulse rate is proportional to the intensity of ultraviolet light, in the range of approximately 250 to 400 nanometers, emitted by a flame. These scanners are particularly suited for monitoring gas flames since the emission from gas flames can be primarily in the ultraviolet range, with only minimal visible light emissions. Ultraviolet flame scanners based on Geiger mueller tubes require extensive maintenance and have relatively limited operational lives as well as unsafe failure modes.
Another type of flame scanner is a photodiode flame scanner. Photodiode flame scanners are the most prevalent type of flame scanner in use today in industrial application. In these flame scanners, visible light, in the range of approximately 400 to 675 nanometers, is collected from inside a combustion chamber, transmitted through a fiber optic cable, and directed onto a single photodiode to produce an electrical signal utilized by the separate processing electronics. Photodiode flame scanners are well suited for monitoring oil and coal flames, as emissions from such flames are in the visible and near infrared ranges.
Flames produced by the burning of different types of fuels have different characteristics. For example, a flame produced by burning a first fuel (a first flame type) might produce one color light, i.e., light in one portion of the spectrum, while a flame produced by burning a second fuel (a second flame type) might produce another, different, color light, i.e., light in a different portion of the spectrum. Conventional flame scanners do not differentiate between, or even recognize, different colors. That is, conventional flame scanners ‘see’ in black-and-white.
However, one conventional flame scanner is known that can recognize an oil flame when oil flames and coal flames are present. However, this flame scanner cannot, at the same time, recognize the coal flame. Thus, this flame scanner is somewhat useful for monitoring oil flames, but limited in monitoring coal flames.
Many modern combustion chambers burn two types of fuels, such as a dual coal and oil burner system. Additionally, a gas- or oil-fired ignitor may be typically used as an ignition source for the main fuel(s). Thus, it is not uncommon for multiple types of flame scanners, one for each type of fuel, to be utilized together. It should be noted that the types of fuels are not limited to oil, coal, and natural gas. Other types of fuels whose flames are monitored include, but are not limited to, black liquor and waste gas fuels.
Utilizing multiple types of flame scanners results in higher initial capital outlays, as well as increased maintenance costs. If a single flame scanner could detect flames produced by multiple types of fuels, fewer flame scanners would be required, reducing both capital and maintenance costs. Accordingly, a need exists for a flame scanner that can detect flames produced by multiple types of fuels.